Concentration of organic acids



Patented Oct. 23, 1951 UNITED STATES PATENT OFFICE CONCENTRATION OF ORGANIC ACIDS No Drawing. Application December 17, 1949, Serial No. 133,686

14 Claims.

1 This invention relates to the concentration of organic acids and relates more particularly to a novel process for the concentration of dilute lower aliphatic acids.

The concentration of dilute aqueous lower aliphatic acids by simple distillation is a tedious operation'necessitating a large consumption of were relatively eiiective in concentrating lower aliphatic acids of about to 40% concentration, such as are obtained during the esterification of cellulose, they were not satisfactory for concentrating acids havinga lower concentration, say less than about 10%, such as are obtained by fermentation processes, for example. At these low acid concentrations, there was a reduction in the distribution coefllcient of the lower aliphatic acids between the solvent or solvent mixture and the dilute acids; with the result that extremely large quantities of the solvents or solvent mixtures were necessary to obtain an adequate recovery of the lower aliphatic acids. In addition to the reduction in the distribution coefllcient at low acid concentrations, there was the further disadvantage that large quantities of the solvents or solvent mixtures had to be distilled to recover relatively small quantities of acids.

It is an important object of this invention to provide a novel process for the concentration of dilute lower aliphatic acids which will be free from the foregoing and other disadvantages of the processes hitherto proposed for this purpose and which will be especially efllcient and inexpensive in operation.

A further object of this invention is the provision of a process for the concentration of dilute lower aliphatic acids by solvent extraction employing as a solvent a mixture of a trialkyl phosphate and a hydrophobic diluent capable on distillation of forming with water an azeotrope having a boiling point below the boiling point of the acids.

Other objects of this invention will be apparent from the following detailed description and claims.

In accordance with our invention, the concentration of dilute aqueous lower aliphatic acids is eilected by extracting said acids with a solvent mixture containing a trialkyl phosphate together with a hydrophobic diluent capable on distillation of forming with water an azeotrope having a boiling point below the .boiling point of the acids. The solvent phase is then separated from the water phase and distilled, the hydrophobic diluent and the water that dissolves in the solvent mixture coming off together as an azeotrope. After the water and hydrophobic diluent have been separated from the solvent phase, the lower aliphatic acids, which have a boiling point appreciably below that of the trialkyl phosphates, are distilled from the solvent phase and may be recovered in highly concentrated or substantially anhydrous form. We have discovered that the solvent mixture containing the trialkyl phosphate and the hydrophobic diluent is particularly efiective in concentrating dilute lower aliphatic acids having a low acid concentration since, as the acid concentration in the aqueous phase is reduced, the.

distribution coefllcient of the acid between the solvent mixture and the aqueous phase is increased.

Suitable trialkyl phosphates that may be employed in preparing the solvent mixture for carrying out the process of our invention are. for example, tripropyl phosphate, tributyl phosphate, tri (butoxy-ethyl) phosphate,tri (2-ethyl-hexyl) phosphate and tri (chlor-ethyl) phosphate. 01 these, the tripropyl phosphate and the tributyl phosphate are preferred since they are from about 1.5 to 2 times as effective as the other trialkyl phosphates in extracting the lower aliphatic acids.

As stated, the solvent mixture contains, in addition to the trialkyl phosphate, a liquid hydrophobic diluent-capable on distillation of forming with water an azeotrope having a boiling point below the boiling point of the lower aliphatic acids. The hydrophobic diluent, which may be present in an amount of from about 25 to by volume, acts to reduce the quantity of water that dissolves in the solvent mixture during the extraction. Moreover, the hydrophobic diluent permits the water that does dissolve in the solvent mixture to be separated therefrom readily by azeotropic distillation at a temperature below the boiling point of the acids. The hydrophobic diluent also acts either to reduce or increase materially the specific gravit of the solvent hase, thus making for easier separation of the solvent and water phases during the extraction. The hydrophobic diluent should be present in the solvent mixture in an amount at least suflicient or preferably slightly in excess of that required to form, on distillation, an azeotrope with all the water a batch basis or in a continuous manner, and

may be carried out in a single stageor in a plurality of stages with concurrent or counter-current flow of the two phases in a manner well known in the art. The extraction may be effected at normal temperatures or at reduced or elevated temperatures. as desired. The volume of the solvent mixture employed during the extraction will depend, in general, upon the manner in which theextraction is carried out, the particular lower aliphatic acids being extracted, the original concentration of the lower aliphatic acids and the permissible concentration of the lower aliphatic acids in the aqueous phase after the extraction thereof is complete.

Following the extraction, the solvent phase containing the major portion of the lower aliphatic acids is separated from the aqueous phase in any convenient manner, taking advantage of the immiscibility and the difference in specific gravity between the two phases. In certain cases, as, for example, during the extraction of the lower aliphatic acids from fermentation liquors, the two phases tend to form an emulsion that does not separate readily on standing The emulsion so formed may be broken by passing the same through a filtering medium such as paper, for example, or in any other manner well known in the art. The aqueous phase remaining after the separation of the solvent phase contains dissolved therein a small proportion of the trialkyl phosphate, say less than about 0.1% by weight. If desired, the major portion of this trialkyl phosphate may be recovered by extracting the aqueous phase with the hydrophobic diluent that is to be employed for preparing the solvent mixture.

The solvent phase containing the lower aliphatic acids is distilled, the water dissolved therein and the hydrophobic diluent coming off together as an azeotrope, which, upon cooling, divides into two layers that may be separated readily and the hydrophobic diluent recovered for'further use. The lower aliphatic acids are then distilled from the solvent phase, coming or! in highly concentrated or substantially anhydrous form. By distilling the lower aliphatic acids from the high boiling solvent phase, the recovery of the acids requires the distillation of only about to of the solvent phase. This represents a marked saving over the prior practice since, when the solvent mixture was distilled from the solvent phase, distillation of about 60 to 80% of this phase was required. During the distillation of thelower aliphatic acids, there is some tendency for the trial-bl phosphates to hydrolyze and liberate free alcohols. which react with the lower aliphatic acids to form esters. The hydrolysis of the trialkyl phosphates and the formation of esters will reduce the yield of the desired products and necessitatethe continuous replenishment of the trialbl phosphates. To minimize or elimihate the hydrolysis of the trialkyl phosphates,

the distillation of the lower aliphatic acids is preferably carried out at a temperature below about 150 C. by effecting the distillation at subatmospheric pressure,-or in any other desired manner. In addition, the trialkyl phosphates are held at the distillation temperature for a minimum period of time by employing a vacuum still of the flash" type, for example, into which the heated solvent phase is passed immediately after the water and hydrophobic diluent have been distilled therefrom. when the lower aliphatic acids contain a strong acid that will cata- Lvze the hydrolysis of the triallgvl phosphates. it is desirable to add a buffer to the solvent layer before distilling the same, in an amount sumcient to destroy the catalytic effect ofthe acid. Suitable strong acid consuming-buffers for this purpose are, for example, the salts of weak acids such as sodium citrate, dibasic sodium phosphate, sodium bicarbonate and sodium acetate.

The process of our invention may advantageously be employed to recover the lower aliphatic acids produced by the fermentation process disclosed in copending application S. No. 133,687, filed on even date herewith. It may also be employed for the concentration of dilute lower aliphatic acids obtained from other sources.

The following examples are given to illustrate this invention further.

Example I dividing the concentration of the propionic acid in the solvent phase by the concentration of the propionic acid in the aqueous phase which is in equilibrium with it.

Example II I The process of Example I isrepeated with a 9.0% weight/volume solution of propionic acid in water. The distribution coeilicient is found to be 4.20.

Examplelli The'process of Example I is repeated with a 9.0% weight/volume solution of propionic acid in water and a solvent mixture containing 70% by volume of tripropyl phosphate and 30% by volume of benzene. The distribution coeflicient is found to be 5.41.

Example IV The process of Example I is repeated with a 2.0% weight/volume solution of acetic acid in water. The distribution coemcient is found to be 1.55.

Example V To parts by volume of a 11.7% weight/ volume solution of propionic acid in water there is added 100 parts by volume of a solvent mixture containing 65% by volume of tributyl phosphate and 35% by volume of benzene. The mixture is agitated for 5 minutes at a temperature of 25 C. After separation. the solvent phase is distilled at atmospheric pressure and 37 parts by volume of a benzene-water azeotrope comes over at a temperature of 68.8 C. The solvent phase is distilled further at a pressure of 35 to 43 mm. of mercury and 9.2 parts by weight of 99% propionic acid comes over at a temperature of from 60 to 64 C.

Example VI A by weight solution of mollasses is fermented in a continuous system for 72 hours with Propionibacterium arabinosum. At the completion of the fermentation period, the batch is acidified to pH 4 with sulfuric acid and 1200 parts by volume of the fermentation mixture is extracted with 300 parts by volume of a solvent mixture containing65% by volume of tributyl phosphate and 35 by weight of benzene. The emulsion which forms during the extraction is broken by passage through a coarse filter paper and the solvent and aqueous phases are separated. The

solvent phase is shaken with 0.2 part by weight To 50-parts by volume of a 2.85% weight/volume solution of propionic acid in water there is added 50 part by volume of a solvent mixture containing 70% by volume of tributyl phosphate and by'volume of ethylene chloride. The mixture is agitated for 5 minutes at a temperature of 25 C. After separation, the solvent phase contains 2.35% weight/volume of propionic acid and the aqueous Phase 0.35% weight/volume of propionic acid, giving a distribution coeilicient of 6.5.

It is to be understood that the foregoing detailed description is given merely by way of illustration and that many variations may be made therein without departing from the spirit of our invention.

Having described our invention, what we desire to secure by Letters Patent is:

1. In a process for the concentration of dilute aqueous lower aliphatic acids, the step which comprises extracting the dilute lower aliphatic acids with a solvent mixture containing a trialkyl phosphate and a hydrophobic diluent capable on distillation of forming with water an azeotrope having a boiling point below the boiling point of the acids.

2. In a process for the concentration of dilute aqueous lower aliphatic acids, the steps which comprise extracting the dilute lower' aliphatic acids with a solvent mixture containing a triallwl phosphate and a hydrophobic diluent capable on distillation of forming with water an azeotrope having a boiling point below the boiling point of the acids, separating the solvent phase from the aqueous phase, distilling from the solvent phase the hydrophobic diluent and the water dissolved in the solvent phase, and distilling from the solvent phase the lower aliphatic acids.

3. In a process for the concentration 01' dilute aqueous lower aliphatic acids, the steps which comprise extracting the dilute lower aliphatic acids with a solvent mixture containing a trialkyl phosphate and a hydrophobic diluent capable on distillation of forming with water an azeotrope having a boiling point below the boiling point of the acids, separating the solvent phase from the aqueous phase, distilling from the solvent phase the hydrophobic diluent and the water dissolved in the solvent phase, and distilling from the solvent phase the lower aliphatic acids at a temperature below about C.

4. In a process for the concentration of dilute aqueous lower aliphatic acids, the steps which comprise extracting the dilute lower aliphatic acids with a solvent mixture containing a trialkyl phosphate and about 25 to 50% by volume of a hydrophobic diluent capable on distillation of forming with water an azeotrope having a boiling point below the boiling point of the acids, separating the solvent phase from the aqueou phase, distilling from the solvent phase the hydrophobic diluent and the water dissolved in the solvent phase, and distilling from the solvent phase the lower aliphatic acids.

5. In a process for the concentration of dilute aqueous lower aliphatic acids, the steps which comprise extracting the dilute lower aliphatic acids with a solvent mixture containing a trialkyl phosphate and at least a sufllcient quantity of a hydrophobic diluent capable on distillation of forming with water an azeotrope having a boiling point below the boiling point of the acids to form an azeotrope with all of the water that dissolves in the solvent phase, separating the solvent phase from the aqueous phase, distilling from the solvent phase the hydrophobic diluent and the water dissolved in the solvent phase, and distilling from the solvent phase the lower aliphatic acids.

6. In a process for the concentration of dilute aqueous lower aliphatic acids containing a strong acid, the steps which comprise extracting the dilute lower aliphatic acids with a solvent mixture containing a trialkyl phosphate, and a hydrophobic diluent capable on distillation of forming with water an azeotrope having a boiling point below the boiling point of the acids, separating the solvent phase from the aqueous phase, adding a butter to the solvent phase, distilling from the solvent phase the hydrophobic diluent and the water dissolved in the solvent phase, and distilling from the solvent phase the lower aliphatic acids.

7. In a process for the concentration of dilute lower aliphatic acids, the step which comprises extracting the dilute lower aliphatic acids with a solvent mixture containing a trialkyl phosphate selected from the group consisting of tripropyl phosphate and tributyl phosphate and a hydrophobic diluent capable on distillation of forming with water an azeotrope having a boiling point below the boiling point of the acids.

8. In a process for the concentration of dilute aqueous lower aliphatic acids, the steps which comprise extracting the dilute lower aliphatic acids with a solvent mixture containing a trialkyl phosphate selected from the group consisting of tripropyl phosphate and tributyl phosphate and a hydrophobic diluent capable on distillation of forming with water an azeotrope having a boiling point below the boiling point of the acids, separating the solvent phase from the aqueous phase, distilling from the solvent phase the hydrophobic diluent and the water dissolved in the solvent phase, and distilling from the solvent phase the lower aliphatic acids.

9. In a process for the concentration of dilute aqueous lower aliphatic acids, the steps which comprise extracting the dilute lower aliphatic acids with a solvent mixture containing a trialkyl comprise extracting the dilute lower aliphatic acids with a solvent mixture containing a triaikyl phosphate selected from the group consisting of tripropyl phosphate and tributyl phosphate and about 25 to 50% of a hydrophobic diluent capable on distillation of forming with water an azeotrope having a boiling point below the boiling point of the acids, separating the solvent phase from the aqueous phase, distilling from the solvent phase the hydrophobic diluent and the water dissolved in the solvent phase, and distilling from the solvent phase the lower aliphatic acids.

11. In a process for the concentration of dilute aqueous lower aliphatic acids, the steps which comprise extracting the dilute lower aliphatic acids with a solvent mixture containing a trialkyl phosphate selected from the group consisting of tripropyl phosphate and tributyl phosphate and at least a sufllcient quantity of a hydrophobic diluent capable on distillation of forming with water an azeotrope having a boiling point below the boiling point of the acids to form an azeotrope with all of the water that dissolves in the solvent phase, separating the solvent phase from the aqueous phase, disiilling from the solvent phase the hydrophobic diluent and the water dissolved in the solvent phase, and distilling from the solvent phase the lower aliphatic acids.

. 12. In a'process for the concentration of dilute aqueous lower aliphatic acids containing a strong acid, the steps which comprise extracting the dilute lower aliphatic acids with a solvent mixture containing a trialkyl phosphate selected from the group consisting of tripropyl phosphate and tributyl phosphate and a hydrophobic diluent capable on distillation of forming with water an aleotrope having a boiling point below the boiling point of the acids, separating the solvent phase om the aqueous phase, adding a butter to the solvent phase, distilling from the solvent phase the hydrophobic diluent and the water dissolved in the solvent phase, and distilling from the solvent phase the lower aliphatic acids.

13. In a process for the concentration of dilute lower aliphatic acids, the step which comprises extracting the dilute lower aliphatic acids with a solvent mixture containing a trialkyl phosphate selected-from the group consisting of tripropyl phosphate and tributyl phosphate and a hydrophobic diluent selected irom the group consisting of benzene, toluene, cyclohexane, methylene chloride, trichlorethylene and 1,2-dichloroethane and capable on distillation of forming with water an azeotrope having a boiling point below the boiling point of the acids.

19;. In a process for the concentration of dilute aqueous lower aliphatic acids, the steps which comprise extracting the dilute lower aliphatic acids with a solvent mixture containing a trialkyl phosphate selected from the group consisting oi tripropvl phosphate and tributyl phosphate and a hydrophobic diluent selected from the group consisting of benzene, toluene, oyclohexane, methylene chloride. trichlorethylene and 1,2-dichloroethane and capable on distillation 01' forming with water an azeotrope having a boiling point below the boiling point of the acids, separating the solvent phase from the aqueous phase, dis- .tilling from the solvent phase the hydrophobic diluent and the water dissolved in the solvent .phase, and distilling from the solvent phase the lower aliphatic acids.

FRED FURTESS. ARNOLD J. ROSENTHAL. BLANCHE B. WHITE.

REFERENCES CITED The following references are of record in the die of this patent:

UNITED STATES PATENTS Number Name Date 2,156,344 Martin Ma 2, 1989 2,165,438 Aliquist July 11, 1939 2,211,087 Weimann Aug. 13, 1940 2,255,235 Wentworth Sept. 9, 1941 2,335,511 Haveniann et a1. Nov. 30, 1915 OTHER REFERENCES Ennis et al., Science, vol. 103, p. 476 (1946). 

1. IN A PROCESS FOR THE CONCENTRATION OF DILUTE AQUEOUS LOWER ALIPHATIC ACIDS, THE STEP WHICH COMPRISES EXTRACTING THE DILUTE LOWE ALPHATIC ACIDS WITH A SOLVENT MIXTURE CONTAINING A TRIALKYL PHOSPHATE AND A HYDROPHOBIC DILUENT CAPABLE ON DISTILLATION OF FORMING WITH WATER AN AZEOTROPE HAVING A BOILING POINT BELOW THE BOILING POINT OF THE ACIDS. 